An optical telecommunication system transmits information from one place to another by way of a carrier whose frequency is in the visible or near-infrared region of the electromagnetic spectrum. Such a carrier is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal. Optical fibers transport the lightwave signal, each of which includes several channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. Multiple channels are commonly transmitted over the same optical fiber to take advantage of the unprecedented capacity offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap. Typically, hundreds or thousands of channels are interleaved by a multiplexer, launched into the optical fiber, and separated by a demultiplexer at a receiver. Along the way, channels may be added or dropped using add/drop multiplexers (ADM) or switched using optical cross-connects (OXC).
Wavelength division multiplexing (WDM) facilitates propagation of multiple channels in a single optical fiber. Wavelength division demultiplexing elements separate the individual wavelengths using frequency-selective components such as optical gratings, which can provide high reflectivity and high wavelength selectivity with the aim of increasing the transmission capacity of optical fibers. One such optical grating is a Bragg grating (e.g. in fiber or in planar waveguides), which selectively transmits or reflects specific wavelengths of light propagating within the optical fiber.
A Bragg grating is a portion of an optical fiber or planar waveguide that has a refractive index profile, which varies periodically along the length of the optical fiber. The center wavelength profile of a Bragg grating is determined by the following equation:λ=2nΛ  (Equation 1)where λ is the center (or Bragg) wavelength, n is the mean effective refractive index, and Λ is the period of the grating (or grating spacing).
Simple periodic fiber Bragg gratings are known in the art and many different methods have been described for fabricating fiber Bragg gratings. One characteristic of fiber Bragg gratings is that, as Equation 1 indicates, to change the center wavelength profile, one can change the refractive index or the grating spacing. Prior art techniques focus on changing the grating spacing, which is accomplished by changing the interference pattern used to define the grating profile. The interference pattern is changed by changing the inter beam angle between two overlapping interfering ultraviolet (UV) light beams used to expose the optical fiber or by changing a phase mask through which UV light is shined.
Changing the phase mask or the inter beam angle tends to be expensive, cumbersome, and labor intensive, however, especially when trying to fabricate several different types of fiber Bragg gratings for the myriad filtering and other applications in optical communication systems. For example, to fabricate fiber Bragg gratings with different center wavelength profiles the writing apparatus is set to different wavelengths, currently by replacing the phase masks. To write long fiber Bragg gratings the optical fiber is translated on long-travel stages to expose new portions of the photosensitive optical fiber to UV light. Similarly, to write chirped broadband fiber-based gratings chirped masks are generally used and new phase masks are used for each new chirp profile. Additionally, writing individual Bragg gratings into separate optical fibers commonly requires time consuming multiple exposures and extensive handling of optical fibers to control the optical fibers.